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At 500 PM AST (2100 UTC), the center of Tropical Depression Eighteen was located near latitude 10.1 North, longitude 33.9 West. The depression is moving toward the west near 15 mph (24 km/h) and this motion is expected to continue for the next day or so followed by a gradual turn to the west northwest by Friday. Maximum sustained winds are near 35 mph (55 km/h) with higher gusts. Strengthening is forecast over the next several days. The depression is forecast to become a tropical storm by tomorrow, and could be near hurricane intensity by the weekend. The estimated minimum central pressure is 1008 mb (29.77 inches).

The tropical wave that NHC has been monitoring over the last several days has gradually become better organized. The satellite structure in particular is quite impressive for a tropical depression, with both the 1800 UTC subjective Dvorak estimates from TAFB and SAB at T2.5/35 kt. This bigger question, however, was if the system possessed a well-defined closed earth-relative circulation. An ASCAT-C pass from earlier this morning hinted that the circulation was becoming better defined, with the development of westerly low-level winds to the south of the convective shield. This westerly low-level flow is also confirmed by atmospheric motion vectors (AMVs) available from the GOES-16 meso domain over the system. While the low-level circulation may still be somewhat broad, it now appears to be well-defined enough to mark the formation of a tropical cyclone. The initial intensity is set at 30-kt, in agreement with peak wind retrievals of 28-30 kt by the earlier scatterometer data.

The initial motion is estimated to be at 270/13 kt, though this is somewhat uncertain given that the center has only recently formed. An expansive mid-level ridge is located to the north and west of the cyclone, which should maintain its heading toward the west, though with a gradual gain in latitude as the system approaches the western extent of the ridge by day 5. The track guidance is in excellent agreement on this track evolution for the first three days, with just a bit more spread in the guidance thereafter. The ECMWF and its ensemble mean towards the end of the forecast is on the left side of the guidance envelope, while the GFS and HWRF models are currently on the right side. For the first NHC track forecast, I have elected to stay close to the track consensus aids, taking a blend of the HCCA and TCVN aids which are near the middle of the guidance envelope.

The environment ahead of the tropical depression appears quite favorable for intensification. Vertical wind shear is forecast by both the ECWMF- and GFS-based SHIPS guidance to stay at or under 10 kt for the next 3-5 days as the storm traverses warm 28-29 C sea surface temperatures. Most of the guidance responds to this environment by indicating strengthening, and the NHC intensity guidance follows suit, showing a steady increase in intensity throughout the forecast period. While it might take a bit of time for the formative low-level circulation to become vertically aligned with the mid-level center, once that occurs, it is possible a period of rapid intensification could occur during the five day forecast. The forecast intensity by 120 hours (100 kt) is on the higher end of the guidance envelope, but not as high as the latest HWRF or HAFS-B forecasts.